A wide variety of packings for stuffing boxes for all types of piston rods, valve stems, shafts and the like are well-known. For example, graphite tape (such as that sold under the trademark GRAPH-LOCK by Garlock Inc) and braided graphite rings (such as that sold under the trademark GARLOCK STYE #98 sold by Garlock Inc) have been very effective packings in most applications. However, when installed into petrochemical industry valves, due to excessive stuffing box-to-stem dimensional tolerances, some leakage has been recorded. GRAPH-LOCK tape, when re-formed under the amount of load that can be applied in ordinary valve housings in the petrochemical industry, will not cold flow sufficiently to provide effective sealing of up to 1/32" clearances when supplied in its standard density of 1.4 g/cc. GARLOCK STYLE #98, being a soft braided packing, allows sight leakage under most high-pressure applications. U.S. Pat. Nos. 883,534 and 3,227,464 show two examples of prior art packings.
In addition, California has amended its law to require less emissions from petro-chemical pumps and valves. To date, pump and valve stem packings required leakage to lubricate wearing surfaces. When utilizing a carbon-graphite system in such valve stem packings, further lubrication after installation is unnecessary. Most petro-chemical industries schedule a routine preventative maintenance tear down inspection, where valves and pumps are reconditioned, thus extending overall service life. It is not uncommon to remachine a valve stem 1/32" to 1/16" overall to renew the sealing surfaces. During disassembly, the gland is cleaned to remove corrosion and system deposits. Both of these operations alter the sealing surfaces resulting in a leakage path upon reassembly using standard packing materials. Use of conventional packing such as braided constructions, twisted rope, and flexible graphite all have limitations. Braided and twisted constructions are porous and dimensionally unstable. Flexible graphite can be made impervious by compression molding causing severe limitations in material flow and excessive sealing stress.
It is an object of the present invention to provide an extremely flexible, effective, and economical packing system. It is another object to use a system of braided packing and flexible graphite to provide such advantages.
It is a further object of this invention to use two different densities of flexible graphite to control material flow and sealing pressure. It is another object to use a difference in angles between a high-density graphite adapter ring and a low density graphite preform ring to provide a predictable stress and material flow. Under low to moderate stress, the difference in angles will provide deflection of the preform toward the designed sealing surface. Further loading will compress the flexible graphite to a previously calculated sealing density. The initial material flow will fill the gap caused by design, wear, stem refurbishment, and gland distortion.
It is another object of this invention to provide a low density, graphite preform ring having a density of less than 1.1 g/cc and preferably of 0.7 g/cc and having means for maintaining its structural integrity.